The preparation of triorganosiloxy-terminated linear organopolysiloxanes from linear organopolysiloxanes having one Si-bonded hydroxyl group in each terminal unit and silicon compounds containing two triorganosilyl groups in each molecule using phosphonitrilic chloride as a catalyst has been described, for example, in U.S. Pat. No. 4,203,913 to Burkhardt et al.
Transparent elastomers have been described in U.S. Pat. No. 4,008,198 to Krohberger et al, in which highly viscous diorganopolysiloxanes are mixed with a nitrogen containing compound having at least one triorganosilyl group and at least one nitrogen atom linked either directly to a silicon atom or via an oxygen atom, from 4 to 40 percent by weight of hexaorganodisilazane, based on the weight of the viscous diorganopolysiloxanes and from 10 to 150 percent of a silicon dioxide filler having a surface area of at least 50 m.sup.2 /g, based on the weight of the viscous diorganopolysiloxanes.
In contrast to the teachings of the Krohberger et al. reference, applicant has found that linear organopolysiloxanes having terminal triorganosiloxy units which are prepared from hydroxy terminated organopolysiloxanes, have high thermal stability, low electrical conductivity, and will not discolor during storage or corrode metal surfaces when a small amount of hexaorganodisilazane is added to the triorganosiloxy terminated organopolysiloxanes in the absence of essential amounts of a filler having a BET surface area of at least 50 m.sup.2 /g
It is, therefore, an object of the present invention to provide a process for preparing triorganosiloxy-terminated linear organopolysiloxanes from linear organopolysiloxanes having one Si-bonded hydroxyl group in each terminal unit and organosilicon compounds having two triorganosilyl groups per molecule, using less phosphonitrilic chloride as the catalyst than was used heretofore to prepare linear organopolysiloxanes having terminal triorganosiloxy units. Another object of the present invention is to prepare triorganosiloxy terminated linear organopolysiloxanes having the same quality, while using less phosphonitrilic chloride catalyst and employing the same residence time for the reactants in the reactor. Still another object of the present invention is to provide a process for preparing linear organopolysiloxanes having terminal triorganosiloxy units which does not require additives in order to deactivate the phosphonitrilic chloride or reaction products thereof, which promote the condensation and/or equilibration of organopolysiloxanes. Still another object of the present invention is to provide a process for preparing triorganosiloxyterminated linear organopolysiloxanes from linear organopolysiloxanes having one Si-bonded hydroxyl group in each terminal unit and organosilicon compounds having two triorganosilyl groups per molecule, using phosphonitrilic chloride as the catalyst, which will give particularly high space-time yields and provide, in particular, mostly physiologically safe linear organopolysiloxanes having an especially small number of Si-bonded hydroxyl groups. A further object of the present invention is to provide a process for preparing triorganosiloxy terminated linear organopolysiloxanes having a high thermal stability and having a particularly low electrical conductivity. A still further object of the present invention is to provide a process for preparing triorganosiloxy terminated linear organopolysiloxanes which will not become cloudy or discolor during storage, and will not corrode metal surfaces.